Glow discharge millimeter wave detector and method of biasing same

ABSTRACT

A millimeter wave glow discharge detector biased to achieve an improvement in signal to noise ratio of about 20 dB. The glow discharge detector tube comprises three or more electrodes, including an anode biased with respect to a cathode so as to generate a first discharge current which forms a high density plasma of electrons in a specific region of the tube, and a sensing electrode positioned with respect to the cathode such that it is affected by the area of high electron density, and biased to draw a relatively small second discharge current. Incident millimeter wave radiation interacts with electrons in the plasma region of the tube, resulting in changes in the second discharge current and the voltage at the sensing electrode.

United States Patent [191 Farhat et a1.

[ GLOW DISCHARGE MllLLllMETER WAVE DETECTOR AND METHOD OF BIIASIZNG SAME [75] Inventors: Nabil El. Earhat, Philadelphia, Pa.;

Norman S. lKopeilta, Kfar l-laroe, Israel [73] Assignee: University of Pennsylvania,

Philadelphia, Pa.

[22] Filed: Oct. 19, 11972 [21] Appl. No.: 299,002

[52] ILLS. CII 329/157, 315/157, 315/350, 328/210, 330/41 [51] mm. Cl Htlllj 17/36, H03d 1/00 [58] Field of Search... 329/157; 315/350, 157, 159; 328/210, 211; 330/41 OTHER PUBLICATIONS Terman-"Radio Engineers Handbook-1943, pp.

INCIDENT MILLIMETER CONICAL HORN 9 GLOW DISCHARGE TUBE, 8

[4 1 Eeb. 5, 1974 347-352, McGraw Hill Book Co. Inc, New York,

Primary ExaminerAlfred L. Brody Attorney, Agent, or Firm--Paul & Paul [57] ABSTRACT A millimeter wave glow discharge detector biased to achieve an improvement in signal to noise ratio of about 20 dB. The glow discharge detector tube comprises three or more electrodes, including an anode biased with respect to a cathode so as to generate a first discharge current which forms a high density plasma of electrons in a specific region of the tube, and a sensing electrode positioned with respect to the cathode such that it is affected by the area of high electron density, and biased to draw a relatively small second discharge current. Incident millimeter wave radiation interacts with electrons in the plasma region of the tube, resulting in changes in the second discharge current and the voltage at the sensing electrode.

8 Claims, 2 Drawing Figures PAIENFED 51974 CONICAL HORN 9 m w E G R FHU OW WE w 6 [TR T A I PW RT AUE VOD 6 O 5 O 4 O 3 O 2 w 0 5 O O 562 SQSO 1985a DISCHARGE CURRENT mm) GLOW DISCHARGE MILLIMETER WAVE DETECTOR AND METHOD OF EIASING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to gas tube microwave detectors, and more specifically to gas discharge millimeter wave detectors having three or more electrodes and biased to optimize signal to noise ratio.

2. Description of the Prior Art Detection of centimeter microwaves with glow discharge plasmas is known in the prior art. Studies have demonstrated that microwave glow discharge detectors possess useful responsivities comparable to those of other detectors, i.e., contact crystal detectors. However, the spectral response of such prior art detectors has been observed to drop with increased microwave frequency. Detection in the 4-2 millimeter wave band has been described by Severin and Van Nie, A Simple and Rugged Wide-Band Gas Discharge Detector For Millimeter Waves, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-14, pages 43 l436, September, 1966, where the authors disclose detection using a low pressure glow discharge in He and Ne, and measurement of an open circuit responsivity of 50 V/W and a noise equivalent power (NEP) of W/l-lz when the tube was used as a synchronous detector mounted in a cylindrical waveguide component. However, despite the above recognition of this type of detector, its application has thus far been confined to the sensing of strong microwave fields. The relatively high inherent noise characteristic of plasma devices has been the limiting factor in using such detectors with weak microwave fields. There are many technical fields today, such as microwave holography, wherein there exists a great requirement for a millimeter wave detector of low NEP, good responsivity, low cost, good spectral response, and large dynamic range. When the glow discharge tube is biased in the manner of this invention, there is provided a noise suppression of nearly dB, thus providing a detector particularly adapted for low power applications such as in close-range microwave holographic arrays.

SUMMARY OF THE INVENTION It is a primary object of this invention to provide a low cost millimeter wave glow discharge detector which provides a substantial improvement over prior art devices in detecting low power millimeter wave sig nals.

It is'another object of this invention to provide a method for biasing a glow discharge tube which has three or more electrodes, so as to maximize signal to noise ratio when such tube is used as a millimeter wave detector.

It is a further object of this invention to provide a sensitive glow discharge millimeter wave detector particularly suited for use in close-range microwave holographic arrays.

In accordance with the above objectives, there is disclosed a glow discharge detector having at least three electrodes including an anode, a cathode, and a sensing electrode, with the sensing electrode positioned substantially on the opposite side of the cathode from the anode, and the anode biased with respect to the cathode such that it draws a large discharge current in the range of 20-45 ma, thereby generating a high space charge density in the region of the sensing electrode, and the sensing electrode being biased to draw less than 1 milliamp, such biasing arrangement improving the signal-to-noise ratio (S/N) of the voltage sensed at the sensing electrode by approximately 20 dB.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows the glow discharge tube and biasing arrangement for same, as comprise the apparatus of this invention.

FIG. 2 is a curve illustrating the variation of glow discharge detector (GDD) output with discharge current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, there is shown a schematic diagram of a glow discharge tube biased in a manner to make it an efficient millimeter wave detector. The glow discharge tube 8, when biased in accordance with this invention, is referred to as a glow discharge detector (GDD). A glow discharge tube, or glow lamp which has been found to be suitable for use as a millimeter wave detector is the Signalite TRQ250 cold cathode triode, a subminiature neon trigger tube. This tube is ordinarily used for precision triggering applications, and in fact was specifically designed by its manufacturer for reliable use in close tolerance electronic applications where switching circuitry is used, particularly where light indication is desired. In the subsequent description of the detector of this invention, and method of biasing same, all performance data relate to the use of this particular tube. However, it is to be noted that other designs of tubes may be used within the scope of the invention as claimed, as will be readily apparent from the discussion of the way the detector tube is biased and operated.

The tube 8 is conveniently placed within a conical horn 9, which horn receives incident millimeter wave energy and focuses same into the area of the tube itself. The tube has an anode connected to terminal A, a cathode connected to terminal C, and a third electrode connected to terminal B as shown in FIG. I. In normal operation of the Signalite tube as a trigger tube, the third electrode is the trigger. In operation of the tube as a microwave detector, such third electrode is referred to as the sensing electrode.

In the bias arrangement shown in FIG. I, the cathode terminal is connected directly to ground, or any suitable reference. The anode is biased to carry a relatively high current, with a suitable voltage source V A in series with suitable limiting resistor R1 connected between the anode terminal and the cathode terminal, so 'as to provide a discharge current from anode to cathode. Suitably, V may be a regulated DC power supply in the order of 200 volts, with R11 chosen to control the anode discharge current within the range of about 20 to 45 milliamps (ma). The sensing electrode is biased by the combination of voltage source V in series with fixed resistor R2 and variable resistor R3, and connected between terminals B and C so as to cause a sec- 0nd discharge current 1 of a magnitude within the range of about 0.1 to 1.0 ma, and passing between the sensing electrode and the cathode. Suitably, V is a regulated DC power supply of about 250 volts. The combination of R2 and R3 is chosen to limit the current to the 3 desired range, with R3 being a potentiometer of suitable resistance to provide operator control of current 1 Terminal B is connected through switch in series with coupling capacitor 11 to a tuned amplifier, or other suitable processing circuit.

In operation, the relatively high current 1,, generates a high electron density within a portion of the tube 8. This area of high electron density is referred to as a space charge. 1n the configuration of the TRQ250 tube, the anode is displaced away from the axis of the tube, while the cathode is cylindrical and placed roughly along the longitudinal axis of the tube. The space charge, or plasma, generated by 1,, exists not only between anode and cathode, but also curves and laps around the centrally located cathode. The sensing electrode B is displaced to one side of the cathode and generally opposite from the position'of the anode, but within the region of the plasma. The sensing electrode, biased to draw very little current, remains relatively clean and insensitive to noise and the destructive effects suffered by most electrodes which are operated at high current densities over substantial periods of time. Although the noise generated by a normal glow discharge generally decreases with increased discharge current, the opposite appears to be true when the tube is operated in the abnormal glow region. Thus, the biasing arrangement of this invention generates such a high charge density as is required to achieve useful responsivity in the detection of millimeter waves, while at the same time maintaining the sensing electrode as a low current drawing electrode, or floating probe. When millimeter wave radiation is directed at the detector tube, the incident amplitude 'modulated radiation is believed to interact with electrons in the space charge regions of the discharge, resulting in an increase in electron temperature which is manifested by a small change in the sensed current 1 or in a change in the floating potential of terminal B (when the resistance of R2 and R3 approximates infinity).

The circuit, biased as shown in FIG. 11, has been tested to determine the optimum biasing conditions for achieving the best signal detection. The GDD was subjected to a fixed incident millimeter microwave radiation, the anode discharge current 1,, was varied, and the detector current 1 was held fixed at 0.9 ma. With these conditions, the following dependence of detected output on anode current was observed:

IA (ma.) Vuut (v01ts) The above data is plotted in FIG. 2, where it is observed that above about ma, the detector output rises greatly, levelling off at about 40 ma, and falling off values of 1,, above about 45 ma. It is seen from this curve that it is important to bias the tube so that the discharge current 1,, is in the range of about -50 ma, in order to achieve the maximum sensitivity of the detector. This range is referred to as the tubes maximum sensitivity range, and is bounded at the lower end by the level of 1,, above which detector output rises sharply, and at the upper end by the level of 1,, above which detector output drops off.

A similar test was made wherein both' incident power and 1,, were held constant, and the detector discharge current 1 was varied. It was found that the dependence of detector sensitivity upon 1 was less critical, but that the sensitivity did rise at about 0.8-0.9 ma, level off, and then drop above about 1 ma. The region of levelling off is referred to as the knee, and is the preferred region in which to operate. Consequently, it is important in the operation of the detector of this invention that the anode be biased to conduct a discharge current 1,, in the range of about 20-45 ma, and the sensing electrode be biased to conduct a discharge current 1,, in the range of about 0.1 to 1 mat, and preferably around 0.9 ma, or at the knee of the sensing electrode response curve.

In operation, it has been observed that the SNR of the voltage sensed at the sensing electrode is improved by nearly 20 dB when 1,, equals 25 ma, as compared to the case of 1,, equal zero orjust a few ma. Thus, an improvement in SNR of at least about 20 dB is accomplished by the biasing arrangement of this invention.

The GDD of this invention has been operated and tested in the synchronous detection mode, i.e., used as a synchronous detector by adding to the incident amplitude modulated millimeter wave signal a signal from a local oscillator which is synchronous with the carrier of the amplitude modulated signal. In such mode of operation, the detector tube was fitted with a 16 dB gain conical horn. The detector was subjected to millimeter wave illumination from a millimeter wave klystron oscillating at 66.8 Gl-lz and from a strong local oscillator. Synchronous detection conditions were realized by positioning the glow discharge detector assembly such that the detected signal was maximized, to ensure that the local oscillator field and the signal field were in phase. With the local oscillator power incident on the detector of 1.5 mW, the noise equivalent power (NEP) of the detector, as measured at a wavelength of 4.5 mm, was found to be about 3 X 10 W/Hz.

The responsivity of the detector of this invention is equal to or better than an ordinary millimeter wave crystal such as the 1N53 used in a conventional crystal mount. Unlike crystal detectors which are subject to burn-out when exposed to high power, the glow discharge detector displays no deterioration in performance at high levels. The sensing electrode stays clean because of the small current which it draws, and the detection characteristics of the detector do not change. Further, the dynamic range, or range of incident power level over which the operation of the device remains linear, has been found to be greater than 30 dB. This means that in the presence of local oscillator power (for example, mW), the noise equivalent power of the detector can be aslow as 3 X 10' W/l-lz. The responsivity (R) of the detector when used in this synchronous detection mode is defined as the detected voltage change observed at the sensing electrode divided by the square root of the product of the local oscillator and modulated signal powers, and for the circuit shown in FIG. 1 was found to be 0.177 V/W.

The detector of this invention has been found to give excellent results in detection of microwave power in the millimeter range, and particularly from 2 to 5 millimeters. Such tubes, when biased as taught herein to produce a high charge density plasma and a low detector current, can detect microwaves with submillimeter wavelengths down to 40 microns. However, since ordinary glass would be opaque to such high frequencies, in order to use the detector of this invention for same it is necessary to encapsulate the tubes in a different type of material which provides a suitable window. Ex-

amples of such suitable materials for use in high frequency windows are found in the proceedings of the IRE, September, 1959, page 1,541. Among such materials are sodium chloride, cadmium sulfide, potassium chloride, and cessium iodine. Also, the material known by the trademark Teflon is adaptable for such use.

It is also to be noted that this invention is not limited to the type of glow discharge tube used or the precise kind of gas used therein. As long as the glow discharge tube is suitable for operation in the abnormal glow region, and can sustain a high anode discharge current over a long lifetime, the tube is suitable. Also, it is to be understood that more than one sensing electrode, and/or more than a total of three electrodes, may be used.

Of greatest importance is the fact that the device of this invention is extremely inexpensive as compared to presently used devices. Applicants detector employs a tube which sells for about 82 cents, and gives performance as a narrow band detector every bit as good as crystal detectors which cost in the range of 400 to 500 dollars. This dramatic improvement is obtained from the biasing arrangement as discovered by applicants and taught herein. As seen from the preferred embodiment using the T110250 tube, it is most important that the detector arrangement involve the following conditions:

a. The tube must sustain a high discharge current, generating a plasma within the tube.

b. The sensing electrode is to be placed within the region of the plasma.

c. The sensing electrode is to draw a minimal current consistent with long lifetime usage, so as to keep said sensing electrode clean and minimize change in operating characteristics. This is done by biasing the sensing electrode at the knee of its response curve.

(1. The high discharge current must be maintained within the maximum sensitivity range wherein, inde' pendent of incident power and current drawn by the sensing electrode, the detected millimeter wave signal is maximized.

The high discharge current 1,, is obtained by operating the tube in the abnormal glow region of the tubes characteristics, where the noise is known to increase with discharge current. This would result in a high noise penalty if the detected signal were sensed at either of the two electrodes between which the high discharge current is drawn. By using a third electrode bi ased to draw low current, and locating it in the region of the plasma (space charge), a high SNR is achieved. The high space charge suppresses noise, as in temperature limited thermionic diodes. In addition, the low current drawn by the sensing electrode assures low shot noise penalty and maximizes the useful tube lifetime.

We claim:

1. Improved glow discharge detector apparatus for detection of millimeter wave signals, comprising:

a. a glow discharge tube, having an anode, a cathode and a sensing electrode, and respective terminals connected thereto;

b. first biasing means connected between the anode terminal and the cathode terminal, for biasing said tube to conduct a first discharge current between said anode and said cathode which is a relatively high discharge current producing a high electron density plasma within said tube;

c. second biasing means connected between the sensing electrode terminal and the cathode terminal, for biasing the tube to conduct a second discharge current between said sensing electrode and said cathode, said second discharge current being relatively low compared to said first discharge current;

d. said sensing electrode being affected by said plasma such that when millimeter wave signals are incident upon said tube a signal representing said incident radiation is developed at said sensing electrode; and

e. output means connected to said sensing electrode for providing an output signal representative of said millimeter wave signals.

2. The apparatus as described in claim 1, wherein said first biasing means is adapted to control said first discharge current to a value within its maximum sensitivity range.

3. The apparatus as described in claim 2 wherein said second biasing means is adapted to control said second discharge current to a value at about the knee of its response curve, so as to optimize the performance of said apparatus in terms of signal sensitivity and lifetime of operation.

4. The apparatus as disclosed in claim ll wherein said first biasing means is adapted to control said first dis charge current to a value within its maximum sensitivity range, and said second biasing means is adapted to control said second discharge current to a value at about the knee of its response curve, so as to optimize the detector sensitivity of said apparatus and its lifetime performance.

5. The apparatus as described in claim wherein said sensing electrode is displaced on an opposite side of said cathode from said anode, and within the plasma region generated by said first discharge current.

6. The apparatus as described in claim 5 wherein said first biasing means is adapted to control said first discharge current to a value within 20-45 ma, and said second biasing means is adapted to control said second discharge current to a value within O.1l ma.

7. A method of operating a glow discharge tube as a millimeter wave detector, said tube having an anode, cathode, and sensing electrode, comprising:

a. biasing said tube between the anode and cathode to produce a first discharge current of a relatively high value, and within its maximum sensitivity range;

b. biasing said tube to produce a second discharge current from said sensing electrode to said cathode and having a value at about the knee of the sensing electrode operating characteristic; and

c. connecting said sensing electrode to an output cir cuit for providing an output signal representative of millimeter waves incident upon. said glow discharge tube.

d. Improved glow discharge detector apparatus for detection of wave signals in the millimeter and submillimeter range, comprising:

a. a glow discharge tube, having at least three electrodes and respective terminals connected thereto and adapted to receive said wave signals when incident thereon;

b. first biasing means connected between a first and a second of said electrodes, for biasing said tube to conduct a first discharge current between said first 7 8 and second electrodes which is a relatively high disthird electrodes which is relatively low compared charge current producing a high electron density to said first discharge current; and plasma within said tube; d. output means connected to said third electrode for c. second biasing means connected to a third of said providing an output signal representative of said electrodes for biasing said tube to conduct a secincident wave signals. 0nd discharge current between said second and 

1. Improved glow discharge detector apparatus for detection of millimeter wave signals, comprising: a. a glow discharge tube, having an anode, a cathode and a sensing electrode, and respective terminals connected thereto; b. first biasing means connected between the anode terminal and the cathode terminal, for biasing said tube to conduct a first discharge current between said anode and said cathode which is a relatively high discharge current producing a high electron density plasma within said tube; c. second biasing means connected between the sensing electrode terminal and the cathode terminal, for biasing the tube to conduct a second discharge current between said sensing electrode and said cathode, said second discharge current being relatively low compared to said first discharge current; d. said sensing electrode being affected by said plasma such that when millimeter wave signals are incident upon said tube a signal representing said incident radiation is developed at said sensing electrode; and e. output means connected to said sensing electrode for providing an output signal representative of said millimeter wave signals.
 2. The apparatus as described in claim 1, wherein said first biasing means is adapted to control said first discharge current to a value within its maximum sensitivity range.
 3. The apparatus as described in claim 2 wherein said second biasing means is adapted to control said second discharge current to a value at about the knee of its response curve, so as to optimize the performance of said apparatus in terms of signal sensitivity and lifetime of operation.
 4. The apparatus as disclosed in claim 1 wherein said first biasing means is adapted to control said first discharge current to a value within iTs maximum sensitivity range, and said second biasing means is adapted to control said second discharge current to a value at about the knee of its response curve, so as to optimize the detector sensitivity of said apparatus and its lifetime performance.
 5. The apparatus as described in claim 4 wherein said sensing electrode is displaced on an opposite side of said cathode from said anode, and within the plasma region generated by said first discharge current.
 6. The apparatus as described in claim 5 wherein said first biasing means is adapted to control said first discharge current to a value within 20-45 ma, and said second biasing means is adapted to control said second discharge current to a value within 0.1-1 ma.
 7. A method of operating a glow discharge tube as a millimeter wave detector, said tube having an anode, cathode, and sensing electrode, comprising: a. biasing said tube between the anode and cathode to produce a first discharge current of a relatively high value, and within its maximum sensitivity range; b. biasing said tube to produce a second discharge current from said sensing electrode to said cathode and having a value at about the knee of the sensing electrode operating characteristic; and c. connecting said sensing electrode to an output circuit for providing an output signal representative of millimeter waves incident upon said glow discharge tube.
 8. Improved glow discharge detector apparatus for detection of wave signals in the millimeter and submillimeter range, comprising: a. a glow discharge tube, having at least three electrodes and respective terminals connected thereto and adapted to receive said wave signals when incident thereon; b. first biasing means connected between a first and a second of said electrodes, for biasing said tube to conduct a first discharge current between said first and second electrodes which is a relatively high discharge current producing a high electron density plasma within said tube; c. second biasing means connected to a third of said electrodes for biasing said tube to conduct a second discharge current between said second and third electrodes which is relatively low compared to said first discharge current; and d. output means connected to said third electrode for providing an output signal representative of said incident wave signals. 